The present invention relates to an optical module having a structure in which an input side and an output side disposed to be separated from each other by a free space are optically coupled to each other through an optically functional portion inserted between the input side and the output side. In more detail, it relates an optical module in which at least one of the input side and the output side has a plurality of collimators at least one of which is made different from the other collimators in at least one variable characteristic parameter to thereby achieve optimum optical coupling between the input side and the output side. For example, the present invention is useful for a matrix optical switch, an optical multiplexer/demultideplexer, an optical tap, and so on.
As well known, a collimator is used in an optical technology in order to convert divergent light from an exit surface of a light-emitting device or an optical fiber to approximately collimated beams, or contrariwise in order to converge the collimated beams on an incident surface of a photodetector or an optical fiber. When two or more collimators as described above are disposed and an optical device of any kind is inserted in a free space through which the magnified beams pass, an optical module can be formed.
In such an optical module, in many cases, the beam diameter may be small and the free space lengths between channels or ports may different from one another in accordance with the channels.
In the background art, however, coupling characteristic of the collimators is not optimized in such a case. For example, a collimator array had a structure in which a required number of collimators of the same kind were simply arranged side by side at regular intervals (so as to make the distance between optical axes of adjacent collimators equal to one another).
If an optical module is configured by use of collimators of the same kind in the case where free space lengths between channels or ports are different from one another, it is however difficult to achieve low insertion loss and low insertion loss deviation even within a light diffraction limit. There, however, arises a problem that low insertion loss and low insertion loss deviation is hardly achieved even in a diffraction limited of the light so that the size of the optical module can hardly be reduced. This problem is caused by the fact that optical coupling cannot be optimized in accordance with channels or ports, the fact that loss is constrained severely so that further light diffraction loss due to small beams used cannot be allowed, the fact that small-size collimators cannot be used in spite of channels or ports small in free space, and so on.
An object of the present invention is to provide various kinds of optical modules small in size, low in insertion loss and low in insertion loss deviation in the case where free space lengths are different from one another between channels or ports.
According to the present invention, there is provided an optical module constituted by a light input side, a light output side disposed to be separated by a free space from the input side, and an optically functional portion inserted between the input side and the output side so that the input side and the output side are optically coupled to each other through the optically functional portion. In the present invention, at least one of the input side and the output side includes a plurality of collimators; and at least one of the collimators is made different from the other collimators in at least one variable characteristic parameter selected from parameters such as a distance between a focal point of a lens and a light exit or incident surface, a numerical aperture of the light exit or incident surface, an effective focal length of the lens, a wavelength used and a distance between optical axes of adjacent ones of the collimators, so that size and position of a beam waist on the input side are made approximately coincident with those on the output side. Here, the light exit surface means a light-emitting surface of a light-emitting device, an end surface of an optical fiber, or the like, whereas the light incident surface means a photodetection surface of a photodetector, an end surface of an optical fiber, or the like.
In the case where a plurality of optical fiber collimators each constituted by a combination of a lens and an optical fiber are provided on at least one of the input side and the output side, at least one of the fiber collimators is made different from the other fiber collimators in at least one variable characteristic parameter selected from parameters such as a distance between a focal point of a lens and an end surface of the optical fiber, a mode field diameter or numerical aperture of the optical fiber, an effective focal length of the lens, a wavelength used and a distance between optical axes of adjacent ones of the fiber collimators, so that size and position of a beam waist on the input side are made approximately coincident with those on the output side.
Hence, such a plurality of fiber collimators may be provided only on the input side or only on the output side or maybe provided on both the input side and the output side. When the plurality of fiber collimators are provided only on the input side, the output side may be constituted by a combination of a collimator lens and a photodetector. When the plurality of collimators are provided only on the output side, the input side may be constituted by a combination of a light-emitting device and a collimator lens. Alternatively, configuration may be made without using any fiber collimator so that the input side is constituted by a combination of a light-emitting device and a collimator lens whereas the output side is constituted by a combination of a collimator lens and a photodetector.
For example, there is a configuration in which the plurality of collimators disposed on at least one of the input side and the output side have a lens array constituted by a plurality of gradient index rod lenses. In the case of having such a configuration, either or each of opposite end surfaces of the lens array is provided as an oblique surface with respect to a direction of arrangement of the array to thereby adjust the variable characteristic parameter. Further, in the case where the plurality of fiber collimators are disposed on at least one of the input side and the output side, there is a configuration in which the plurality of fiber collimators are constituted by a combination of a lens array of a plurality of gradient index rod lenses and a fiber array of a plurality of optical fibers. In the case of having such a configuration, either or both of opposite end surfaces of the lens array and/or an end surface of the fiber array are provided as oblique surfaces with respect to a direction of arrangement of the array to thereby adjust the variable characteristic parameter. Further, it is effective to allocate longer wave to a channel longer in free space length.
Specifically, for example, collimators are provided on each of the input side and the output side, the collimators on the output side are disposed at an angle of 90 degrees with respect to the collimators on the input side, and a movable mirror is inserted in a position corresponding to each channel in the free space, so that a matrix optical switch can be formed. In such a case, it is desirable to have a configuration in which points of intersection between optical axes of the input side and optical axes of the output side are set to be equivalent to lattice points of a square lattice, movable mirrors are disposed at the lattice points respectively, and adjustment is made so that beam waists are formed on a diagonal line of the square lattice.
In the configuration in which the input side and the output side disposed to be separated from each other by a free space are optically coupled to each other through an optically functional portion inserted between the input side and the output side, an optical device constituted by either of a filter and a semi-transparent mirror is used as the optically functional portion, and adjustment is made in such a manner that beam waists as equal to one another as possible are formed on the optical device. In such a manner, an optical multiplexer/demultiplexer or an optical tap can be formed.
For example, a fiber collimator with two fibers is disposed on one side whereas a fiber collimator with single fiber is disposed on an opposite side to the side, a filter is provided as the optical device in a free space, one of optical fibers in the fiber collimator with two fibers is provided as an input side whereas the other optical fiber is provided as an output side, an optical fiber in the collimator with single fiber is provided either as an input side or as an output side, and adjustment is made in such a manner that beam waists as equal to one another as possible are formed on a filter surface. In such a manner, an optical multiplexer or an optical demultiplexer can be formed. Further, a fiber collimator with two fibers is disposed on one side whereas a fiber collimator with single fiber is disposed on the opposite side, a semi-transparent mirror is provided as an optical device in a free space, one of optical fibers in the fiber collimator with two fibers is provided as an input side whereas the other optical fiber is provided as an output side, an optical fiber in the collimator with single fiber is provided as an output side, and adjustment is made in such a manner that beam waists of the fiber collimators as equal to one another as possible are formed on the semi-transparent mirror. In such a manner, an optical tap can be formed.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-303813 (filed on Oct. 3, 2000), which is expressly incorporated herein by reference in its entirety.